I see one reasonable option for spending such a large part of income on gambling - if a person has huge savings and current income does not have much influence on his life, then if he wants, he can even spend 100 percent on gambling. All other options look dubious to me, although I am not saying that this is an addiction, just a wrong decision (in my subjective opinion).
Only those person who has a huge savings and current income are for sure who has also the guts to over spend or lets say it is only a part of their excess money. For them it was an excess money but for us or personally speaking for me a 20% of income to allot for my entertainment or gambling activities is such a huge amount. We may take over spending as gambling addiction which is definitely true but for me I look for myself a sign of not having a good sleep at night while keep on thinking of my gambling activities.
Wrong decision and addiction is just pretty much same in my opinion, unless you never try it again, because if your wrong decision keeps on repeating, that's definitely an addiction without a doubt. Losing 100% of your income is insanely bad for you since you'll have no more for yourself as well as your bills. You cannot say to your bills to wait because you've lost your money in gambling, that's very not valid.